This invention relates to a knock sensor for accurately detecting knocking of an engine for automotive vehicles or the like.
The occurrence of engine knock in an automobile is an important factor which is detrimental to the smooth running of the automobile, the durability of the engine and the improvement of the fuel consumption. Such knocking can be eliminated by controlling the ignition timing, the air-fuel mixture ratio or the like.
FIGS. 1a and 1b show the vibration waveforms of a vehicle engine detected by a vibration pickup having a flat frequency characteristic. While a high-level waveform A appears periodically in response to each ignition in both the normal operating condition of FIG. 1a and the knocking condition of FIG. 1b, in the knocking condition another high-level waveform B appears at a time different from the time of each ignition in addition to the periodic waveforms A.
FIGS. 2a and 2b show the spectra of the waveforms shown in FIGS. 1a and 1b. FIG. 2a corresponds to the normal operating condition and FIG. 2b corresponds to the knocking condition. As will be seen from FIGS. 2a and 2b, in the normal operating condition the spectrum has no marked characteristic feature, although there are some irregularities. In the knocking condition, however, the spectrum shows some characteristic high peaks at 7 to 8 kHz. Such peak varies depending on the type and size of engines and it is present at 6 to 9 kHz. As a result, the knocking condition can be detected from the level of the peak in this band. On the other hand, the S/N ratio of a signal detected in the vicinity of a vehicle engine is extremely inferior electrically. Consequently, if the vibration waveform is simply picked up by a vibration pickup having a flat frequency characteristic, it will be impossible to efficiently detect a frequency which is characteristic of the knocking.